Finally, the SSMEs can be throttled, or their thrust varied, due to the ability to control fuel flow. This is necessary to maintain a steady acceleration, since the mass of the shuttle decreases rapidly during ascent as it burns fuel; two minutes into the flight, the shuttle weighs less than half of what it did at launch. It is also used to manage dynamic pressure on the orbiter during the launch in order to not exceed structural load limits; fortunately, maximum dynamic pressure (called Max Q in shuttlespeak) is reached fairly early. Then atmospheric pressure begins to drop off rapidly, allowing the engines to be spooled up; this is what the phrase "Go for throttle-up" indicates during the ascent. During peak output, the SSMEs put out 109%of their rated thrust; a 5% safety margin means that maximum power used is 104%. This is possible because their 100% power output level was a fixed specification during the design phase of the Orbiter, but Rocketdyne produced engines which output 109% of that power level; NASA retained the power rating as designed and introduced the 5% safety margin.